1. Field of the Invention
The present invention relates to an optical pickup device and an optical disk apparatus mounted on electronic equipment, such as on a personal computer.
2. Description of the Related Art
Conventionally, DVDs (Digital Versatile Discs) have only a single layer data recording face on one side. Recently, however, to cope with recent increases in memory capacities and to enable data recording and reproduction, DVDs having two layer data recording faces on one side have become available on the market. FIG. 37 is a diagram showing the configuration of an optical system for a conventional optical pickup device. A light source 501 is a DVD laser light source, and included for it is a light emission point for a DVD to which a laser beam having a wavelength λ (of about 650 nm) is emitted. A prism 502 has an internal slope for which a polarizing separation film is provided. A collimating lens 503 alters the laser beam emitted by the light source 501 to provide an almost parallel beam. A beam splitter 504 reflects most of the laser beam emitted by the light source 501, and permits only a part of the laser beam to pass. The beam splitter 504 also reflects almost all the light reflected by an optical disk 500. A path elevating prism 505 is a prism that changes the laser beam path from a route substantially parallel to the optical disk 500 to one that is almost upright. A hologram element 506 is a combination of a hologram 506a and a quarter-wave plate 506b. In accordance with the polarized state of a laser beam, the hologram 506a passes the laser beam emitted by the light source 501, and separates the laser beam reflected by the optical disk 500 into light fluxes required for focus control and for tracking control. The quarter-wave plate 506b changes the polarized light of the laser beam emitted by the light source 501 from P polarized light to circularly polarized light, and changes the polarized light of the laser beam reflected by the optical disk 500 from circularly polarized light to S polarized light. The hologram element 506 is arranged in a lens holder, together with an object lens 507, and is movable in both the focal and the tracking directions. The object lens 507 changes the parallel light of the laser beam, which is a result provided by the collimating lens 503, to converged light, so that the light can be condensed at the optical disk 500. A light-receiving sensor 508 receives a portion of the laser beam reflected by the optical disk 500, alters the beam to produce various electric signals, and then outputs these signals. A fore light monitor 509 receives part of the laser light emitted by the light source 501 and passed through the beam splitter 504, and changes the laser beam into an electric signal in accordance with the quantity of light. This electric signal is employed as light quantity control for the light source 501.
An example division pattern for the hologram 506a is described in JP-A-2005-63621. FIG. 38A is a diagram showing example division areas within a division pattern for a conventional hologram. FIG. 38B is a diagram showing an example wherein many more division areas are formed. And FIG. 38C is a diagram showing an example wherein only peripheral portions are defined as division areas. According to JP-A-2005-63621, the hologram 506a is substantially divided into four areas by a division line, extending in the radial direction, that is parallel to the radius of the optical disk 500, and a division line 506c, extending in the tangential direction, that is tangentially parallel to the circumference of the optical disk 500. Further, the division areas are divided into areas for tracking detection and areas for focus detection. At the least, some of the areas for tracking detection are defined as division areas 506d along the tangentially aligned division line in the direction of the optical disk 500. In FIGS. 38A, 38B and 38C, only the division line 506c in the tangential direction and the division areas 506d are shown. The division areas 506d are symmetrically located along the division line 506c, and when light fluxes from the division areas 506d are employed as auxiliary light for tracking control, a tracking control signal can be stably obtained, even when a far-field pattern provided by the laser beam emitted by the light source 501 is shifted. Furthermore, when many more division areas 506d are established, as shown in FIGS. 38B and 38C, an adverse affect on light reflected by the optical disk 500, the result of a scratch thereon, could also be dispersed.
FIG. 39A is a diagram showing the state of an optical disk having a two layer data recording face wherein a laser beam is reflected by the front data recording face. FIG. 39B is a diagram showing the state wherein a laser beam is reflected by the rear data recording face. When a laser beam 510 is condensed and reflected by an L0 layer 500a, which is a front data recording face, part of the laser beam 51 passes through the L0 layer 500a and is reflected by an L1 layer 500b, which is the rear data recording face. Further, when the laser beam 510 is condensed and reflected by the L1 layer 500b, part of the laser beam 510 is reflected by the front L0 layer 500a. Therefore, when the laser beam 510 is condensed and strikes the L0 layer 500a, or is further condensed and strikes the L1 layer 500b, the part of the laser beam that has been reflected but has not been condensed is again reflected. Subsequently, the non-condensed and reflected laser beam enters a light-receiving sensor 508, together with the condensed and reflected laser beam that contains a signal element. Therefore, the element of the non-condensed and reflected laser beam is added as an offset to the signal that is output by the light-receiving sensor 508.
Therefore, according to JP-A-2002-190132, provided are a photo detector for 0-order diffracted light that is generated by the hologram 506a, a focus servo signal generation/operation circuit, a photo detector for ±1-order diffracted light and a focus servo signal generation/operation circuit. As a result, focal control can be stably performed. Further, according to JP-A-2003-67949, a diffraction grating that generates three beams is provided between a light source 501 and a prism 502, and the quantity of ±1-order light that has been condensed and reflected is equal to or greater than the quantity of 0-order light that has not been condensed and reflected. Therefore, while for the output of the light source 501 there is a slight loss, the offset for a tracking control signal can be reduced.
Assume that data recording or data reproduction is to be performed by condensing a laser beam at a predetermined data recording face of an optical disk having a plurality of layers, e.g., that data recording is to be performed for the L0 layer of an optical disk having a two layer data recording face and is then to be sequentially performed for the L1 layer. In this case, when the data recording state for other than the predetermined data recording face is unbalanced, an offset for a tracking control signal will occur.